Wide range continuously tunable thin film laser

ABSTRACT

A thin film of laser material is prepared from two different compounds known to have lasing capabilities at different wavelengths and also having one component in common. The two compounds are caused to be deposited in a thin film layer of varying concentration so as to produce a final thin film of lasing material which changes in composition along a selected axis from a layer of laser material comprising substantially wholly one of the compounds, through varying gradient degrees of relative concentrations of the two compounds to a final gradient which may comprise substantially wholly the other compound of laser material. A source of excitation energy is provided, together with means for producing relative motion between the thin film of excitation energy so that the laser output is continuously tunable over an extremely broad band of outputs.

United States Patent 91 Smiley WIDE RANGE CONTINUOUSLY TUNABLE THINFILMLASER [75] Inventor: Vern N. Smiley, San Diego, Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

[22] Filed: July 18, 1969 [2]] Appl. No.: 842,914

OTHER PUBLICATIONS Volume Excitation of an Ultrathin Single-Mode CdSeLaser; Stillman et al.;.Applied Physics Letters; Oct. 66; Vol.9, No.'pg. 268-pg. 268-269. End-Pumped Laser Emission from cadmium Sulfide 51July 17, 1973 Selenide Bombarded by an Electron Beam; Jour. App.

Physics; Vol. 38; June 1967; Tait et al.; pg. 3035-3036.

[57] ABSTRACT A thin film of laser material is prepared from twodifferent compounds known to have lasing capabilities at differentwavelengths and also having one component in common. The two compoundsare caused to be deposited in a thin film layer of varying concentrationso as to produce a final thin film of lasing material which changes incomposition along a selected axis from a layer of laser materialcomprising substantially wholly one of the compounds, through varyinggradient degrees of relative concentrations of the two compounds to afinal gradient which may comprise substantially wholly the othercompound of laser material. A source of excitation energy is provided,together with means for producing relative motion between the thin filmof excitation energy so that the laser output is continuously tunableover an extremely broad band of outputs.

4 Claims, 3 Drawing Figures WIDE RANGE CONTINUOUSLY TUNABLE THIN FILMLASER STATEMENT OF GOVERNMENT INTEREST The invention described hereinmay be manufactured and used by or for the Government of the UnitedStates of America for governmental purposes without the payment of anyroyalties thereon or therefor.

CROSS REFERENCES TO RELATED APPLICATIONS BACKGROUND OF THE INVENTIONAlthough a number of laser devices of varying configuration andarrangements have been devised employing relatively thin films of lasermaterial in one degree or another, none is known at present toembracethe capability of continuous tuning over a large spectral width of laseroutput. Some of the known and prior art thin film lasers are capable ofproducing alaser beam in a direction parallelto the principal plane ofthe thin film, while others are known to be capable under certainconditions of producing a laser beam in a direction perpendicular to theprincipal plane of the thin film laser material, affording the desirablecharacteristic of a relatively large laser beam aperture. However, ofall the knownlaserswhich may be considered to be tun- 'able,the,span ofsuch tunability is relatively very small, covering only very narrowspectral bandwidths which typically may be of only a few Angstrom unitsin the visible spectral region, at most. Applicant has devised a verywide band, continuously tunable, thin film laser which may readily bedevised to be continuously tunable over more than 1,000 Angstrom unitsin the visible spectral region.

SUMMARY oF THE INVENTION The concept of the present inventioncontemplates a "thin film of laser material which may comprise twocompounds that may be designated as AB and AC. These two compounds havedifferent resonant lasing frequencies which may be separated by a fewthousand Angstrom units, for example. The film of laser materialcomprising these two compounds is caused to be created with a varyingconcentration of one of the compounds relative to the other of thecompounds in the thinfilm of laser material and may extend from oneexing the composite compound deposit as being of the form AB,C, where xis the relative concentration of B. When x =1, the composite comprisessubstantially wholly the AB compound but when x 0, the composite issubstantially wholly the AC compound. Accordingly, x may be considered'as the concentration index and when x is made to vary with distancealong a principal axis'of the thin film of laser material, thatconcentration index is a function of distance along the gradient axis.

A source of excitation energy is provided and appropriate means areadapted to produce relative movement between the thin film of lasermaterial along its gradient axis and the source of excitation energy sothat the frequency of the lasing output from thethin film of lasermaterial varies substantially from the resonant lasing frequency of oneof the two compounds, through a changing gradient, to the resonantlasing frequency of the other of the two compounds employed in the thinfilm laser material. Thus, the concept of the present inventionprovidesa continuously tunable laser output which may be of the order of 1,000Angstrom units or more in the visible spectral region in a typicalembodiment.

Accordingly, it is a primary object of the present invention to providea narrow bandwidth continuously tunable laser.

Another most importantobject of the present invention is to provide anarrow bandwidth continuously tunable laser operating over a very widespectral region andin a single mode of operation.

Another important object of the present invention is to provide a narrowbandwidth continuously tunable laser operative as a coherent oscillatoror a continuously tunable coherent amplifier.

A further object of the present invention is to provide a narrowbandwidth continuously tunable laser, opera tive over a very widespectral region and in a single mode of operation, which may beexcitedto lasing level with either optical energy, electron beams, orelectron injection.

These and other features, advantages and objects of the presentinvention will be better understood from the following description ofseveral embodiments which are illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially cross-sectional,schematic illustration of an embodiment of the present invention;

FIG. 2 is a partially cross-sectional, schematic illustration of avariant embodiment of the present invention; and

FIG. 3 is a partially cross-sectional, schematic i llustration ofanother variant embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There is illustrated in FIG. 1,an embodiment of the present invention which includes a thin,wedge-shaped film of laser material 10 having coated thereon on eitherside dielectric, or metallic films 11 and 12 possessing a relativelyhigh degree of reflectance over the spectral region of interest. Thethin film of laser material has a small wedge angle and may comprise twocompounds designated as AB and AC for purposes of explanation, where thetwo components involved which are not common to both compounds, i.e., Band C are varied in their relative concentration so that a gradient isproduced, preferably along a major dimension of the plane of the thinlayer of laser material.

If the composite layer is expressed in the form AB C where x is therelative concentration of B, x may be varied from to I. When x l, thecompound is substantially wholly comprised of AB and when x O, thecompound is substantially comprised wholly of AC. In between theseextremes, there is a continuous and gradual variant in the concentrationof B relative to C. If, as is taught by the concept of the presentinvention, the concentration index, x, is made to vary with distancealong a major dimension or axis, such as the plane of the thin film, theconcentration index at will be a function of the distance, z, as shownin FIG. 1, and will have a gradual and continuous gradient along thatdirection.

If it be considered, for example, that the composite thin film employedin a particular embodiment of the present invention comprises CdSe,S,the relative concentration along the axis of the gradient may be made toproduce a change in the resonant frequency of operation when a source ofexcitation energy is employed which can be controllably positionedrelative to the gradient. Assuming the composite layer of laser materialcomprises CdSe,S when x 1, the composite is reduced to wholly CdSe whichhas a peak laser output at about 6,900 A; when x 0, the composite thinlayer is almost wholly CdS which has a characteristic peak laser outputat about 4,900 A. Accordingly, the extremes of variation of the resonantfrequency of the laser material will nominally be between 6,900 A and4,900 A. In addition the wedge angle is such that the cavity resonancealso changes simultaneously so as to be in tune with the materialresonance at all positions along the film axis. This wedge is such thatthe optical path length between reflectors is always a multiple ofone'-half wavelengths of the desired resonant frequency. Whenconcentration of the components in the thin film of laser material varyalong the gradient of the z axis as illustrated in FIG. 1, (that is,where x also has a gradient varying between I and 0) the peak laseroutput will vary commensurately and continuously between 6,900 A and4,900 A. The composite layers 10, 11 and 12 may be appropriatelydeposited on a suitable substrate material (not shown) if desirable.

An appropriate source of excitation energy 13 is provided together witha suitable optical system 14 mounted on a moveable support i5. When theexcitation energy 13, as focused upon the thin film of laser material10, is absorbed in sufficient quantities by the thin film of lasermaterial 10, a laser beam output 16 is produced. The laser beam output16 will therefore have a wavelength which may be expressed in terms ofA, and which will commensurate with the disposition of the excitationenergy impinging upon the thin film laser relative to the gradient axiswhich extends generally along the plane of the thin film oflasermaterial and is a function of the distance 2 as shown in FIG. 1.

In accordance with the concept of the present invention, it is onlynecessary that relative movement between the gradient axis of the thinfilm of laser material 10 and the source of excitation energy impingingthereon be developed to change the resultant resonant frequency of thelaser beam output 16. Thus, the source of excitation energy 13, togetherwith its optical system 14, may be disposed along the gradient axis by acontrollable and moveable support 15 as illustrated in FIG. 1.Alternatively, within the concept and teaching of the present invention,the thin film of laser material 10 may be made to move substantiallyalong its gradient axis relative to a stationary source of excitationenergy, such as that shown at 13, together with its appropriate opticalsystem 14 in FIG. 1.

The material CdSe S, used as an illustrative example for purposes ofexplanation, is a single crystal semiconductor although other materialsmay be utilized. One method of producing the desired linear gradient inthe general case used for illustration employing the composite AB,,C,may be achieved by forming a thin wedge-shaped film by vapor depositionfrom two separate sources, one containing AB and the other containing ACwhile employing a moving slit mask to continuously vary the rate ofdeposition of AB as compared to the rate of deposition for AC. Inaddition, the total rate of deposition is varied so as to produce acomposite film of continuously varying thickness. In some cases it maybe desirable to recrystallize the deposited films by means of heattreatment.

The thin layer of laser material may be a single layer or may comprisemultiple layers of films deposited on the surfaces of the laser materialas shown in FIG. 1. The two layers of films l1 and 12 are of highreflectance characteristic within the spectral region of interest.Additionally, the layer of laser material 10 is of a generally wedgeconfiguration having a very small wedge angle. It will be appreciated bythose skilled and knowledgeable in the art that this device then becomesa FABRY-PEROT wedge interference filter whose resonant wavelength may beexpressed by the relation N Ares 2nd where N is the order of thicknessin terms of halfwavelengths, n is the index of refraction of the lasermaterial, and d is the actual thickness at any position along the filmplane. The variation in A, must just match the variation in peakwavelength of emission of the laser material. When this condition ismet, the laser will oscillate at any desired wavelength within themaximum obtainable range when suitably excited or energized at theappropriate position 2 as illustrated in FIG. 1.

The laser excitation may comprise focused optical energy or a smallaperture electron beam. The optical energy which is produced by thesource of excitation energy 13 as illustrated in FIG. 1 may comprise agas discharge device of a non-coherent character, such as a Xenon lamp,or coherent light as produced by another laser. As previously mentionedand explained, a continuously variable output of wavelength may beobtained by continuously varying the relative positions of the source ofexcitation energy and the thin film of laser material. When electronbeam excitation is used no coatings will be employed on that surface onwhich the beam impinges.

The high reflectance coatings 11 and 12 as illustrated in FIG. 1, mustbe of broadband character so that they offer relatively high reflectanceover the entire range of wavelengths of operation of the laser yet mustoffer low reflectance to the excitation wavelengths. The laser materialitself, as illustrated at 10 in FIG. 1, must be thin enough to ensuresingle mode operation. That is,

: Aw if 2nd 2nd where it, and A are two neighboring resonances and A0 isthe mean wavelength of oscillations. The optical thickness nd then mustbe less than,

For example, ifltty 6:6() A antith' lasermaterial has a bandwidth of 100A, the maximum thickness of the wedge-shaped laser material must be lessthan 24 comprises a wedge-shaped Fabry-Perot laser cavity. For thegeneral case employing a composite deposition AB C for illustrativepurposes, the desired gradient film of laser material may be achieved byheating a thin wafer of AB such that a temperature gradient along the zdirection as illustrated in FIG. 2 is established while the wafer is inan atmosphere which is rich in C. For example, a thin wafer ofcrystalline CdSe heated with a temperature gradient in an atmosphere ofH 8 or other 10 vapor rich in sulphur will produce the desired results.

FIG. 3 illustrates a variant embodiment of the present invention whereina thin film of laser material is formed in a slightly wedge-shapedconfiguration of the thin film of laser material 10 as is illustrated inFIG. 1. Extremely thin layers of material 31, 32, 33, and 34 selected toform a high reflectance composite on one side of the thin film of lasermaterial 30 and similar extremely thin films of material 35, 36, 37, and38 are formed on the opposite side of the thin film oflaser material 30.Where it is not possible because of the parameters involved in aparticular application of the concept of the present invention to employmultiple layers of substantially quarter wavelength thickness of theemitted laser energy, it may be possible to employ an appropriate singlemetallic film to admit the energy of the electron beam while providing adesirable degree of optical reflectance.

present at' the extreme ends of the wedge-shaped thin film of lasermaterial previously described. Additionally, the coating 12 which is onthat side of the thin film of laser material 10 facing the source ofradiant energy 13 must not offer too high a reflectance to the energyemitted from the excitation source 13.

When the source of excitation energy 13 is optical in nature, itsradiation will be of shorter wavelength than the shortest wavelength oflasing operation of the laser material. It may consist of a non-coherentlight source such as an arc lamp or a gas discharge device;alternatively, a coherent light source such as a single or multiplewavelength laser may be used as the source of excitation energy. Inaddition, multiple photon absorption may be employed' for energizing thelaser of the present invention, whereby two or more photons combine toraise an atom of the laser material to two or more times the energy ofone photon. Accordingly, longer wavelength radiation may be used in thelaser pump or source of excitation energy.

FIG. 2 illustrates a variant embodiment of the present invention. A thinfilm of laser material 20 is supported on a substrate 21 with a film 22of high reflectance character positioned therebetween. The substrate 21and high reflectance film 22 are selected to have satisfactorytransmittance characteristics relative to the excitation energy receivedfrom a suitable source 23. The excitation energy source 23 may comprisea coherent or non-coherent light generator 24 similar to that describedin connection with the illustration of FIG. 1, and having an appropriatelens system 23 to focus the excitation energy.

A second substrate 26 disposed at slight angle relative to the plane ofthe first reflectance film 22, supports a second reflectance film 27 sothat the assembly A source of excitation energy in the form of anelectron beam gun 39 is mounted on a support 40 arranged to be moveablealong the principal axis of the thin film of laser material 30.

The electron beam source 39 pumps the thin film of laser material 30with exciting energy, raising it to a lasing level so as to producelaser beams at 41. The thin film of laser material as illustrated inFIG; 3 has the same characteristics of varying composition inconcentration index along its principal plane axis so that it variesfrom a concentration of substantially wholly one lasing material at oneend, through a gradually and continuously varying concentration of twolaser materials, to a concentration of substantially wholly the otherlaser material at the other end of its principal plane axis.Accordingly, when the thin film of laser material 30 and the source ofexcitation energy 39, in the form of an electron beam, are movedrelative to each other along the principal axis of the thin film oflaser material 30, a continuously tunable laser output is produced whichhas a wavelength commensurate with the composition and film thickness atthe position of the excitation beam relative to the principal plane axisof the thin film of laser material 30.

When an electron beam source of exciting energy is employed asillustrated in FIG. 3, reflectance coatings 31 through 34 positioned onthe side of the thin film of laser material 30 nearest the source ofexcitation energy 39 may be eliminated to ensure that the maximum amountof electron beam energy reaches the laser material. However, it ispossible to use coatings so extremely thin that electrons may betransmitted through while the laser energy is reflected. Desirable,extremely thin reflectance coatings may be accomplished within the stateof the art employing very thin metallic or dielectric coatings.

It will be appreciated by those skilled and knowledgeable in the artthat elements of the three embodiments teaching and concept of thepresent invention. For example, it may be desirable to employ an opticalsource of excitation energy as disclosed in FIG. 1 with the thin film oflaser material and reflectance coatings in the configuration asillustrated in FIG. 3. Conversely, it may be desirable, or necessary inparticular applications, to employ a source of excitation energy such asthe electron beam source 39 illustrated in FIG. 3 with a thin layer oflaser material as illustrated in a configuration as shown in FIGS. 1 or2. Moreover, any desirable number of reflectance coatings on one or bothplane surfaces of the thin film of laser material may be employed withinthe teaching and concept of the present invention. Such coatings exhibitvarying characteristics as may be desirable or necessary for the optimumor proper operation of the laser over particular spectral regionsofinterest and variations of wavelength of laser outputs.

The concept and teaching of the present invention contemplates that thelaser assembly may be used as an amplifier or an oscillator. In theamplifier configuration in addition to the source of excitation energy,an input signal is focused onto one surface of the thin film of lasermaterial and the amplified output signal emits from the other side ofthe thin film of laser material. Regenerative amplification withreflecting surfaces as shown in FIGS. 1, 2, or 3 may be used ortraveling wave amplification may be obtained. In the latter case,antireflective coatings are applied to both surfaces of the thin film oflaser material and a configuration of laser material such as isillustrated in FIG. 3 is employed. In the amplifier configuration, thesource of excitation energy which pumps the laser material (whether itbe an optical energy source or an electron beam) may impinge upon thethin film of laser material at an angle rather than perpendicular to thefilm plane. When the exciting energy is optical in character, the inputsignal and exciting radiation may both be brought into the filmperpendicular to the film plane by the use of a beam splitter; when anelectron beam is employed, the energy may be redirected through a rightangle by a magnetic field. These arrangements allow the input signalfrom the signal source and the energy from the excitation source to beapplied to the laser material without one blocking the other.Alternatively, the source of optical energy or the electron beam may beapplied at an angle to the plane of the thin film of laser material andneed not be applied perpendicular to the base of the device.

From the foregoing descriptions of several embodiments it will beappreciated that the teaching and concept of the present inventionprovides a continuously tunable thin film laser of extremely wide rangenot realized heretofore. Moreover, the reproducibility of the tunedlaser output is enhanced in its precision and the concept of theinvention may be advantageously embodied in a variety of differentconfigurations employing numerous combinations of materials.

In the described preferred embodiments employed for purposes ofillustration and explanation it should be appreciated that theillustrations of FIGS. 1, 2, and 3 are not drawn to scale in theinterests of simplicity and clarity in understanding their respectivemanners of operation. Those skilled and knowledgeable in the art willappreciate that the extremely thin films and coatings referred to in theforegoing explanations are of the order of a relatively few wavelengthsof the energy involved and that the proportions shown in the illustratedembodiments are not intended to be a scalar representations.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A wide range continuously tunable thin film laser for producingsingle mode laser output perpendicular to the principal plane of thefilm comprising:

a thin film of laser material including at least two compounds having acommon element,

said thin film having a gradient varying in thickness in a thin wedgeconfiguration along a major axis in which the relative concentration ofthe uncommon components of said two compounds varies, and having amaximum optical thickness mi of less than A Z/ZAA, where n is theeffective index of refraction of the laser material, d is its actualthickness at any position along the film plane, A is the mean wavelengthof oscillations, and AA is the difference in wavelength between adjacentresonant frequency portions of the film;

a source of excitation energy;

means for focusing said energy upon said film; and

means for causing relative movement between the focused excitationenergy and said thin film along its gradient.

2. A wide range continuously tunable thin film laser for producingsingle mode laser output perpendicular to the principal plane of thefilm comprising:

a thin film of laser material including at least two compounds having acommon element, said thin film having a gradient along a major axis inwhich the relative concentration of the uncommon components of said twocompounds varies, and having a maximum optical thickness rid of lessthan A Z/AA, where n is the effective index of refraction of the lasermaterial, d is its actual thickness at any position along the film palneA is the mean wavelength of oscillations, and AA is the difference inwavelength between adjacent resonant frequency portions of the film; atleast one film of high reflectance in the spectral region of interest onsaid thin fim of laser material,

said films of high reflectance having varying thickness as a function ofdistance along the plane of said thin film of laser material such thatthe maximum reflectance is maintained for all resonant frequencies alongthe plane of said film of laser material within the spectral region ofinterest;

a source of excitation energy;

means for focusing said energy upon said film; and

means for causing relative movement between the focused excitationenergy and said thin film along its gradient.

3. A wide range continuously tunable thin film laser for producingsingle mode laser output perpendicular to the principal plane of thefilm comprising:

a thin film of laser material including at least two compounds having acommon element,

said thin film having a gradient along a major axis in which therelative concentration of the uncommon components of said two compoundsvaries, and having a maximum optical thickness nd of less than A 2/2AA,where n is the effective index of refraction of the laser material, d isits actual thickness at any position along the film plane, A is the meanwavelength of oscillations, and AA is the difference in wavelengthbetween adjacent resonant fre quency portions of the film;

a film of high reflectance in the spectral region of interest on eachside of said thin film of laser material;

a source of excitation energy; 7

means for focusing said energy upon said film; and

means for causing relative movement between the focused excitationenergy and said thin film along its gradient.

4. A wide range continuously tunable thin film laser for producingsingle mode laser output perpendicular to the principalplane of thefilm'comprising:

a thin film of laser material including at least two compounds having acommon element,

said thin film having a gradient along a major axis in which therelative concentration of the uncommon components of said two compoundsvaries, and having a maximum optical thickness mi of less then A Z/ZAA,where n is the effective index of refraction of the laser material, d isits actual thickness at any position along the film plane, A is the meanwavelength of oscillations, and AA is the difference in wavelengthbetween adjacent resonant frequency portions of the film; multiple layerfilms of high reflectance in the spectral region of interest on saidthin film of laser material; a source of excitation energy; means forfocusing said energy upon said film; and means for causing relativemovement between the focused excitation energy and said thin film alongits gradient. i

I read A /ZAA read A /ZA'A-" read plane,

EDWARD M.FLETCHER,JR.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.,747,021

Dated July 17, 1973 Inventor(s) I VERN N; SMILEY It'is certified thaterror appears in theabove-identified patent andthat said LettersPatentare hereby corrected as shown below} Claims 1, 3, and 4, theexpression "A z/2AA should r Claim 2, the expression "X Z/AX'" shouldClaim 2, line ,12, "palne" should Claim 4, line 9 ."then" should readthan Signed and sealed this 22nd day ofdanuary 1974 (SEAL) Attest:

RENE D. TEGTMEYER Att'esting Officer Acting Commissioner of Patents I

2. A wide range continuously tunable thin film laser for producingsingle mode laser output perpendicular to the principal plane of thefilm comprising: a thin film of laser material including at least twocompounds having a common element, said thin film having a gradientalong a major axis in which the relative concentration of the uncommoncomponents of said two compounds varies, and having a maximum opticalthickness nd of less than lambda 02/ Delta lambda , where n is theeffective index of refraction of the laser material, d is iTs actualthickness at any position along the film palne 0 is the mean wavelengthof oscillations, and Delta lambda is the difference in wavelengthbetween adjacent resonant frequency portions of the film; at least onefilm of high reflectance in the spectral region of interest on said thinfim of laser material, said films of high reflectance having varyingthickness as a function of distance along the plane of said thin film oflaser material such that the maximum reflectance is maintained for allresonant frequencies along the plane of said film of laser materialwithin the spectral region of interest; a source of excitation energy;means for focusing said energy upon said film; and means for causingrelative movement between the focused excitation energy and said thinfilm along its gradient.
 3. A wide range continuously tunable thin filmlaser for producing single mode laser output perpendicular to theprincipal plane of the film comprising: a thin film of laser materialincluding at least two compounds having a common element, said thin filmhaving a gradient along a major axis in which the relative concentrationof the uncommon components of said two compounds varies, and having amaximum optical thickness nd of less than lambda 02/2 Delta lambda ,where n is the effective index of refraction of the laser material, d isits actual thickness at any position along the film plane, lambda 0 isthe mean wavelength of oscillations, and Delta lambda is the differencein wavelength between adjacent resonant frequency portions of the film;a film of high reflectance in the spectral region of interest on eachside of said thin film of laser material; a source of excitation energy;means for focusing said energy upon said film; and means for causingrelative movement between the focused excitation energy and said thinfilm along its gradient.
 4. A wide range continuously tunable thin filmlaser for producing single mode laser output perpendicular to theprincipal plane of the film comprising: a thin film of laser materialincluding at least two compounds having a common element, said thin filmhaving a gradient along a major axis in which the relative concentrationof the uncommon components of said two compounds varies, and having amaximum optical thickness nd of less then lambda 02/2 Delta lambda ,where n is the effective index of refraction of the laser material, d isits actual thickness at any position along the film plane, lambda 0 isthe mean wavelength of oscillations, and Delta lambda is the differencein wavelength between adjacent resonant frequency portions of the film;multiple layer films of high reflectance in the spectral region ofinterest on said thin film of laser material; a source of excitationenergy; means for focusing said energy upon said film; and means forcausing relative movement between the focused excitation energy and saidthin film along its gradient.